This invention relates to zinc sulfide-based electroluminescent phosphors and more particularly to a method for improving the life of such phosphors.
Phosphors are a known class of materials that emit electromagnetic radiation, usually in the visible region of the spectrum, upon excitation by another form of energy. Phosphors are usually named for the type of energy to which they susceptible. For example, cathodoluminescent phosphors are excited to emission by impingement of electrons or cathode rays; photoluminescent phosphors are excited to emission by impingement of actinic radiation; x-ray phosphors by the impingement of x-rays; and electroluminescent phosphors by the action of an electric field, usually supplied by an alternating current. It is with the latter type of phosphor that this invention is most particularly concerned.
Commercial electroluminescent phosphors are mostly based upon a host material of zinc sulfide activated by one or more activators among which are copper and/or manganese and/or chloride, which are incorporated into the host structure. These phosphors have uses as lamps or illuminating sources for readout devices or watch faces. Two basic kinds are employed, one in which the phosphor is dispersed in an organic binder and laminated in plastic, and a second variety in which the phosphor is dispersed in a ceramic dielectric and fused to a metal plate.
The plastic variety is the more economical; however, the light output of the phosphor encased in an economical plastic such as Mylar(trademark) degrades rapidly. (The lifetime of an electroluminescent phosphor is described by the xe2x80x9cHalf-Lifexe2x80x9d. The half-life of a phosphor is that point in time when its light output is reduced by half from some initial light output, usually measured after an initial 24 hours).
It would be an advance in the art if the half-life of such phosphors could be extended.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to increase the half-life of electroluminescent phosphors.
These objects are accomplished, in one aspect of the invention, by a method for improving the half-life of a zinc sulfide-base electroluminescent phosphor, comprising the steps of, charging phosphor particles into a reaction vessel; fluidizing the particles with an inert gas; heating the reaction vessel and its contents to 200xc2x0 C.; introducing hexakis(dimethylamido)dialuminum into the reaction vessel; and reacting the hexakis(dimethylamido)dialuminum with the phosphor particles for 10 minutes.